Natural killer (NK) cells were implicated recently in two important biological functions, i.e., in immune surveillance to malignancies and resistance to already established tumors (especially to leukemias) and in rejection of allogeneic bone marrow transplants. Because of the possible involvement of NK cells in these important biological functions, we will: (a) investigate the effect of several immunobiological modifiers on NK cell-mediated cytotoxicity and their mechanism of action; of special interest will be to study various types of interferons (various recombinant interferon clones and immune interferon) and interferon inducers (especially those of the pyrimidinole group); these agents were selected because of their potential therapeutic benefit in cancer patients; (b) study and to determine the cause underlying the low NK cell activity in some of the normal individuals and the NK cell defect in leukemic patients and to attempt to correct these NK cell defects by NK cell modulation; (c) attempt to determine diagnostic, prognostic and therapeutic value of NK cells in leukemia, its regression and relapse; (d) investigate the involvement of NK cells in rejection of allogeneic bone marrow grafts and (e) attempt to develop new matching techniques for bone marrow transplantation on the basis of NK cell activities.